Seasonal controls on the diet, metabolic activity, tissue reserves and growth of the cold-water coral Lophelia pertusa

Vast cold-water coral (CWC) reefs occur in temperate regions, where strong seasonality in temperature and light leads to a short but highly productive spring period. How CWCs respond physiologically to this strong seasonal forcing remains unclear, due to the remoteness of their deep-sea habitats. In an in situ transplantation study at Nakken reef, Norway, we investigated a full seasonal cycle of (1) temperature and food availability, (2) diet, (3) biomass and tissue reserves, (4) oxygen consumption and (5) linear growth of the reef-building coral Lophelia pertusa. All investigated variables showed a distinct seasonality. An increase in the organic carbon and amino acid content, linear extension and budding rate from February to late May, at a simultaneous increase in phytoplankton and zooplankton fatty acid trophic markers (FATMs), and delta N-15-derived trophic level, indicates an efficient exploitation of the spring phytoplankton and the subsequent zooplankton bloom. A pool of neutral-lipid-derived fatty acids, indicative of energy storage and gametogenesis, was formed from May to October, accompanied by increased oxygen consumption, i.e. metabolic activity. In late autumn and early winter (October-December), tissue reserves were maintained, in spite of low sPOM and zooplankton food availability, and the lower tissue delta C-13 and higher contribution of bacterial FATMs suggest increased reliance on more degraded material. The concurrent reduction in linear growth further suggests a lower energy availability at this time of the year. A large (> 50%) drop of all tissue pools between December and February coincided with the spawning season of L. pertusa and demonstrates a high energetic cost of reproduction. Our results show for the first time a strong seasonal control of critical life history traits such as growth patterns and timing of reproduction in this prominent deep-sea species.